Ch 73 - Osteochondrosis Flashcards

1
Q

What are the main processes of endochondral ossification?

A
  • Matrix mineralisation
  • Chondrocyte death
  • Vascularisation
  • Ossification
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2
Q

What are the three main phases of osteochondrosis?

A
  • Osteochondrosis latens - early, microscopic lesion
  • Osteochondrosis manifesta - subclinical lesions which are macroscopically and radiographically apparent
  • Osteochondrosis dissecans - Attached or loose cartilage flaps are present and typically result in clinincal signs
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3
Q

How much longitudinal bone growth is formed from the growth plates and from the epiphysis?

A
  • Growth plate: 75 - 80%
  • Epiphysis: 20 - 25%
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4
Q

What are the widely accepted location of OCD?

A
  • Humeral head
  • Medial aspect of humeral condyle
  • Lateral or medial femoral condyle
  • Medial or lateral trochlear ridge of the talus
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5
Q

What inhibits circumferential expansion of the growth plate?

A

Perichondral ring of Lacroix

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6
Q

What are the 4 zones of the growth plate?

A
  • Resting zone (epiphyseal side)
  • Proliferative zone
  • Hypertrophic zone
  • Mineralisation zone
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7
Q

Which is the only vascularized zone of the physis?

A

The resting zone
- Penetrated by chondro-epiphyseal blood vessels within cartilage canals

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8
Q

Describe the chondrocyte appearance in each of the physeal zones:

A
  • Resting zone: Small, scattered, randomly organized. Primarily slowly dividing stem cells
  • Proliferative zone: Flat, relatively small, organized into COLUMNS. They divide, slowly enlarge and produce matrix
  • Hypertrophic zone: Spheroid and relatively large. Rapidly swell and continue synthesis of matrix
  • Mineralization zone: Newly formed matrix mineralizes and chondrocytes undergo apoptosis
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9
Q

What effect does growth hormone have on the resting zone?

A
  • Promotes differentiation into daughter cells capable of making Insulin-like growth factor 1 (IGF-1) which stimulates clonal expansion of chondrocytes
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10
Q

What substances are involved in the local feed-back loop of chondrocyte proliferation?

A
  • PTHrP
  • IHH (Indian hedgehog)
  • TGF-B

Controls the irreversible differentiation of proliferative chondrocytes into hypertrophic chondrocytes. BMP, thyroid hormone and others are also needed for this phenotypic change to occur.

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11
Q

How does physeal mineralised cartilage undergo endochondral ossification?

A
  • Clasts remove the tranverse matrix septa seperating the apoptotic chondrocytes from the metaphyseal vasculature
  • Clasts form void lacunae in a mineralised matrix scaffold
  • Vasculature from the metaphysis and osteoprogenitor cells invade the lacunae under control of VEGF produced by hypertrophic zone chondrocytes
  • Osteoprogenitor cells differentiate into osteoblasts and produce woven bone which is then replaced by lamellar bone
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12
Q

What are the 2 layers of the articular-epiphyseal complex?

A

Relatively thin outer layer
- Specialised immature articular cartilage, avascular and takes no part in endochondral ossification
- Developes into mature cartilage with 4 zones: Superficial, transitional, radial and calcified cartilage
- Noncalcified radial zone seperated from calcified cartilage by the tidemark. This indicates completion of maturation process

Inner layer functionally similar to the growth plate with 2 main differences:
- Visually disorganised withouh ordered zonal and columnar arrangement of chondrocytes
- Abundant vasculature from the perichondral plexus and course through cartilage canals, forming glomeruli
- Most proliferation occurs at periphery whereas conversion into bone occurs at the center

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13
Q

What is chondroification?

A

Endochondral ossification - the process of regression of cartilage canals as the ossification from the center of the epiphysis reaches newly formed epiphyseal cartilage.

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14
Q

What remains once the process of epiphyseal ossification is finished?

A

A thin layer of avascular articular cartilage and a crtilage disc between the epiphysis and metaphysis (growth plate)

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15
Q

How has microtrauma been speculated to cause OCD?

A

Microtrauma can cause damage to cartilage canal vessels at the chondroosseous junction and subsequent necrosis of cartilage canals leading to areas of cartilage ischaemia and necrosis
- Infarcted cartilage focally prevents endochondral ossification however cartilage proliferation continues, resulting in focal thickening
- Thickened cartilage may be less resistant to mechanical stress and may be metabolically deprved and degenerate
- Weakened cartilage may deform, fissures can form and may propage along tidemark (osteochondral junction)
- If fissures extend to the joint surface, an OCD lesion develops

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16
Q

What is the grading scheme for OCD lesions in the proximal humerus?

A
  • Grade I: Cartilage surface is normal, slightly thickened with miniscule subchondral defect
  • Grade II: Surface mottled, more thickened, small subchondral cleft
  • Grade III: Discoid elevation of cartilage surface, large subchondral cleft, sclerotic subchondral bone
  • Grade IV: Partially detached flap or seperated flap and joint mouse
17
Q

What is the main difference in the manifestation of articular OCD vs growth place osteochondrosis?

A
  • Articular is associated with cartilage necrosis
  • Growth plate is associated with persistence of hypertrophic chondrocytes (thought to result from damage to resting zone and metaphyseal blood supply)
18
Q

What is thought to be the cause of clinical signs with a OCD lesion?

A
  • Joint fluid coming into contact with subchondral bone may elute necrotic cartilage particles or inflammatory mediators and provoke a synovitis
  • Inflammatory mediators within synovial fluid can stimulate nociceptors in subchondral bone
  • Altered loading
19
Q

What are palliative techniques for OCD?

A

Debridement and lavage

20
Q

List the main reparative techniques for surgical treatment of OCD

A
  • Curettage
  • Spongialisation (complete debridement of subchondral bone, not really recommended)
  • Abrasion arthroplasty
  • Forage (aka osteostixis, subchondral drilling) using K-wire or microdrill burr
  • Microfracture using small picks or awls. 3-4mm between each hole
21
Q

List the main restorative techniques for surgical treatment of OCD

A
  • Fragment reattachment (common in humans, not really done in dogs)
  • Osteochondral transplant using single large plugs or mosaicplasty (OATS®, Arthrex). Clinical improvement is expected but a degree of lameness tends to persist
  • Synthetic osteochondral resurfacing (SynACART®, Arthrex) Long-term function excellent, osseous integration 83% with no periarticular reaction or damage on 2nd look arthroscopy
22
Q

What are four risk factors for OCD?

A

Heredity
Rapid growth
Diet
Trauma
(IMPORTANT)

23
Q

Most long bone growth in dogs occurs between what ages?

A

12-26 weeks of age (IMPORTANT)